The launch failure of an unmanned Russian vehicle will likely mean the cancellation of an upcoming crew rotation. If a remedy cannot be found soon, the six aboard the station may have to leave.

The International Space Station could be flown from the ground without… (NASA )

NASA warned Monday that the International Space Station may have to be abandoned at least temporarily later this year, following the launch failure of an unmanned Russian rocket that is also used in part to send astronauts to the outpost.

The accident last week will almost certainly force the cancellation of a scheduled Sept. 21 launch of the Russian Soyuz capsule that was part of a routine crew rotation, said Michael Suffredini, NASA space station chief and chairman of the Space Station Control Board, an international body that runs the program.

Suffredini said the agency and its partners would avoid leaving the station unmanned, if they can. "We will focus on keeping the crews safe," he said.

When NASA retired the U.S. space shuttle system in June, it left the space station partners dependent on Russia for transporting astronauts aboard the decades-old Soyuz craft.

At the time, Suffredini expressed confidence in the Soyuz, and said NASA did not have a contingency plan in place in case the Russians ran into problems.

But Suffredini said that unless the Russians can quickly determine the cause of the rocket failure and implement a remedy, the six-member crew now on board will have to leave over the next three months.

Two Soyuz capsules are docked at the station, each with space to carry three astronauts, so there is no immediate danger to the six crew members.

NASA can operate the station from the ground without a crew, but the risks of such unmanned operations increase by tenfold the probability of a catastrophic loss of the space station. The orbiting laboratory is considered the most expensive machine ever built with a price tag of more than $100 billion.

Three of the astronauts are scheduled to return in September, but daylight conditions at the landing site in Kazakhstan would allow for only a short extension. The other three astronauts would have to return in mid-November.

The Soyuz capsule is certified to operate for 200 days, and that time limit will expire in mid-November for the last three-member crew.

The Progress crashed in a sparsely inhabited forest in Siberia, and Russian teams have not yet located the wreckage, he said.

The Russian space engineers "have acknowledged extra pressure from their government," Suffredini said. But in broad terms, he said the Russians had done "an outstanding job" of transporting crews to the station.

Space shuttle proponents had warned about the risks of relying on the Russians and argued against retiring the fleet. If the space station were lost, the political repercussions could be tremendous.

"A lot is at stake," said Tim Farrar, president of Telecom, Media & Finance Associates, a Menlo Park-based satellite and telecommunications consulting and research firm. "There have always been concerns about the U.S. relying on the Russians. This presents a big opportunity for companies like SpaceX to show they are reliable."

Hawthorne-based rocket venture Space Exploration Technologies Corp., or SpaceX, is planning to send a capsule from Cape Canaveral, Fla., on Nov. 30 to dock with the space station.

The unmanned mission is a demonstration flight to show that the company can pull off the feat.

In December, SpaceX became the first private company to blast a spacecraft into Earth's orbit and have it return intact in a test flight. NASA had originally planned on SpaceX completing another demonstration flight before allowing the company to dock with the station. But now, the space agency has tentatively agreed to combine flights to speed up the process.

SpaceX already has a $1.6-billion contract to haul cargo in 12 flights to the space station for NASA. If the docking mission is successful, the company would start in earnest to begin fulfilling the contract by next year. After that, the company hopes to ferry astronauts.

"The United States should not be entirely reliant on Russia to carry American astronauts to the space station," said Kirstin Grantham, a SpaceX spokeswoman. "We need alternatives for carrying astronauts and we need them as soon as possible."